A prior art flat panel display device, such as an active matrix type liquid crystal display device (AMLCD) includes amorphous silicon thin-film transistors as switching elements of pixels because their active semiconductor layers, such as amorphous silicon films, can be uniformly formed on a large-area optically-transparent substrate at relatively low temperatures. Recently, thin-film transistors for driver circuits have been also formed at circumferences on the same substrate as those for such switching elements. It is disclosed in Japanese Unexamined Patent Publication 2000-187248, and, particularly, at its pages 4 through 6 and FIGS. 1 through 3 that the active semiconductor layers of such thin-film transistors are made from polycrystalline silicon because the mobility of the polycrystalline silicon by electric field effect is greater than that of the amorphous silicon.
Since the substrate of a prior art AMLCD, however, is made of a glass plate which is optically transparent and electrical insulation, static electricity is charged on the substrate. The static electricity may destroy the switching elements or the driver circuits to significantly lower a yield of the AMLCD. The static electricity has been higher as a size of the substrate has been bigger. As a result, electrostatic destruction of the switching elements and driver circuits becomes a more critical problem.
When the substrate charged with static electricity, for instance, is lifted up by pins or is placed on projected pads from a grounded stage, grounded states at portions of the substrate on the pins or pads differ from those of the other portions of the substrate. They cause redistribution of charges of the static electricity so that voltages at the portions of the substrate on the pins or pads are induced several-tens of times or more than those of the other portions of the substrate.
In the case, particularly, that a gate insulation film is formed at small polycrystalline silicon patterns isolated from major ones on the substrate, that long gate-electrode leads are formed on the gate insulation film, and that electric capacitors are formed between the polycrystalline silicon patterns and the gate-electrode leads, such redistribution of charges of the static electricity has a more adverse affect than when the substrate is lifted up by pins or is placed on projected pads from the grounded stage. Electrostatic destruction may occur between the polycrystalline silicon patterns and the gate electrode lines. In other words, it is highly possible for the static electricity to destroy the thin-film transistors.